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# Light - Reflection and Refraction

## Light - Reflection and Refraction Synopsis

Synopsis

Nature of Light

• Light enables us to see several thousands of objects every day.
• Though light is invisible, it makes objects around us visible.
• Light is a form of electromagnetic radiation and a non-mechanical wave.
• Non-mechanical waves do not require a material as a medium for propagation.
• Light travels along a straight line and this property are called the rectilinear propagation.
• This straight line is called a ray, and a bundle of rays is called a beam of light.
• Thus, real images are formed when the rays after reflection or refraction meets at a point.
• If the rays appear to meet when produced backwards, then the image formed is virtual.
• The real image can be caught on a screen but the virtual image cannot be formed on a screen.

Reflection of Light

• When light falls on a body, it may be absorbed, may be transmitted or may return to the same medium.
• In the reflection of light, light waves are neither transmitted nor absorbed, but they are deflected from the surface of the medium back into the same medium.
• This is governed by the laws of reflection.

• First law of reflection: The incident ray, the normal to the surface at the point of incidence and the reflected ray, all lie in the same plane.
• Second law of reflection: The angle of incidence is equal to the angle of reflection.

Characteristics of the Image formed by a Plane Mirror

• The image formed by a plane mirror is always virtual and erect.
• The size of the image is equal to the size of the object, and the image is laterally inverted.
• The image formed by the plane mirror is as far behind the mirror as the object is in front of it.

• Similar to plane mirrors, curved mirrors also form images. Mirrors whose reflecting surfaces are spherical are known as spherical mirrors.

Refraction of Light

• Light travelling obliquely from one medium to another undergoes a change in its direction of propagation.
• The phenomenon of change in the path of light from one medium to another is called refraction of light.

Laws of Refraction

• First law of refraction: The incident ray, the refracted ray and the normal to the interface of the two transparent media at the point of incidence, all lie in the same plane.
• Second law of refraction: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media and for a given wavelength of light. This law is also known as Snell’s law.

• The constant is called the refractive index of the second medium with respect to the first medium.
• The cause of refraction is the change in the speed of light as it goes from one medium to another medium.
• Larger the difference in speed of light, the greater will be the angle of bending and vice versa.

Refractive Index

• The extent of the change in the direction of a light ray that takes place in a given pair of media is expressed in terms of the refractive index.
• Light travels fastest in vacuum and also with almost the same speed in air. In other media, its speed is relatively less. The value of the refractive index for a given pair of media depends on the speed of light in the two media.
• The refractive index of medium 2 with respect to medium 1 is given as;

• Similarly, the refractive index of medium 1 with respect to medium 2 is

• If medium 1 is vacuum or air, then the refractive index of medium 2 is considered with respect to vacuum. This is known as the absolute refractive index of the medium.

Refraction of Light through Glass slab
• Consider a rectangular glass block PQRS.
• A light ray AO falls on the surface PQ. NOM is the normal to the surface PQ at the point of incidence O. At the surface PQ, the ray AO travels from air to glass, so it bends towards the normal NOM and travels inside the glass in a straight-line path along OB.
• At the surface RS, the ray OB suffers refraction again. N1BM1 is the normal to the surface RS at the point of incidence B.

• Ray OB travels from glass to air, so it bends away from the normal and travels along BC.
• The ray AO is called the incident ray, OB the refracted ray and BC the emergent ray. ÐAON is the angle of incidence i, ÐBOM is the angle of refraction r and ÐCBM1 is the angle of emergence e.
• Since refraction occurs at two parallel surfaces PQ and RS, therefore ÐMOB = ÐN1BO and Ði = Ðe, i. e., the angle of incidence is equal to angle of emergence by the principle of reversibility of the path of a light ray. Thus, the emergent ray BC is parallel to the incident ray AO.
• The perpendicular distance XY between the path of emergent ray and the direction of incident ray is called the lateral displacement.

Spherical Mirrors
• A spherical mirror whose reflecting surface is curved outwards and polished on the inner spherical surface is a convex mirror.
• A spherical mirror whose reflecting surface is curved inwards and polished on the outer spherical surface is a concave mirror.

Image Formation by a Concave Mirror for Different Positions of an Object